1. Field of the Invention
The present invention relates to a multiple beam scanning apparatus and a light source unit for a multiple beam scanning apparatus utilized in a high-speed optical writing system for use in a laser printer, digital copying machine, laser facsimile device, etc.
2. Discussion of the Background
To realize high-speed recording, a multiple beam scanning apparatus has been well known as an apparatus capable of simultaneously scanning plural lines of light beams on a photosensitive body and thereby performing image recording. Two types of such multiple beam scanning apparatuses are generally well known. The first type is a beam composing system in which plural semiconductor lasers (laser diodes, hereinafter referred to as a "semiconductor laser") are employed as a light source and light beams emitted from the respective semiconductor lasers are composed by use of a polarization beam splitter, etc. The second type is an array system in which a semiconductor laser array having plural light-emitting points is employed as the light source.
In the case of the array system, it is difficult to procure the semiconductor laser array itself, and if the semiconductor laser array can be procured it is very expensive. In particular, when the number of light-emitting points increases, namely the number of light beams capable of being emitted increases, the difficulty in procurement and costs also increase. Furthermore, in order to obtain a desired beam pitch in the subscanning direction on the photosensitive body, the light-emitting points arranging direction of the semiconductor laser array is set by inclining the direction thereof by a predetermined angle from the main scanning direction.
However, at this time, the greater the number of light beams, the more enlarged the expansion of the beams in the main scanning direction as shown in FIG. 16. FIG. 16, shows a semiconductor laser array 100, a collimator lens 101, a rotatable multiple surface polygon mirror 102, an optical element 103 such as an f.theta. lens, etc., and a photosensitive body 104. In this type of device there arises a drawback that the rotatable polygon mirror 102 and the optical element 103 are inevitably large-sized.
On the other hand, in the case of the beam composing system, although a widely used ordinary semiconductor laser can be employed, the accuracy of adjustment is very strict for obtaining a predetermined distance between adjacent scanning lines on the photosensitive body. In addition, the relative position of the light beam on the photosensitive body largely changes by optical lever action due to any slight variation of the relative position between the semiconductor lasers caused by temperature variation and vibration, etc. Therefore, it is difficult to stably maintain the predetermined scanning lines distance.
Regarding the above-mentioned points, in the beam composing system plural semiconductor lasers are provided so as to be unitarily combined into one unit together with plural collimator lenses and a beam composing medium. Thereby, time-elapsing variation of the beam pitch can be reduced, and a predetermined scanning lines distance can be obtained by rotating the light source unit for such an optical apparatus around its optical axis. Examples of the above-mentioned proposal have been known hitherto, for example, in Japanese Laid-Open Patent Publication No. 7-181412/1955.
According to the description of this background art, the distance between the scanning lines can be easily set and adjusted with high accuracy. However, as recognized by the present inventor, in the case of realizing a multiple beam number equal to or more than three, it is necessary to use a number of semiconductor lasers equal to the beam number, and thereby the light source unit inevitably turns out to be large-sized.